Electrostatic writing and printing device using an electrostatic actuator



June 13, 1967 R. J. WOHL 3,325,676

ELECTROSTATIC WRITING AND PRINTING DEVICE USING AN ELECTROSTATIC ACTUATOR Filed Nov. 26, 1963 DEVELOPMENT 29 GENERAL CONTROL /28 CONTROL CIRCUITRY I 21 218 220 251 o II E 2 E flV 421 404 422 405 5 406 424 FIG.3 FIG.4

INVENTOR. ROBERT J. WOHL WWW ATTORNEY ELECTRQSTATIC WRKTEING AN?!) PRHNTHNG DE- VICE USHNG AN ELECTRGdTATlIC AtITUATUR Robert 33'. Wohi, an .iose, Caiif, assignor to international Business Machines Corporation, New York, Nfifl, a

corporation of New York Filed Nov. 26, 1963, Ser. No. 326,174 4 (Claims. (Cl. 3l5-2l) AESTRAQT UP THE DHSCLQSURE A device for creating an electrostatic image on one side of a target and for applying toner particles to the opposite side of the target. The device for forming the image is a conventional electron gun. The device for applying toner particles includes a member which is electrostatically vibrated to project the particles onto the target.

The present invention relates to electronic display devices and more particularly to an improved apparatus wherein a visible display is generated through the medium of an electrostatic charge.

United States Patent 3,109,062, entitled, Electrostatic \Vriting and Printing Device, by Calvin K. Clauer and Jack D. Kuehler which is assigned to the assignee of the present invention, shows an electrostatic writing and printing device for producing a visible display of information which is represented by a video input signal. In the system shown in the above application an electrostatic image is formed by an electron beam which is directed toward an electrically non-conducting surface which is capable of storing electrostatic charges in small discrete areas thereof. The electrostatic target surface is disposed in one end of an evacuated envelope which has a transparent window or face plate. The target surface is disposed adjacent to but spaced from the transparent window. Disposed in the space between the target surface and the transparent window is a supply of small particles capable of retaining an electrical charge. These particles termed developer are of a suitable color to produce a visible image.

A visible image is produced by the system described in the above cited reference as follows: first, the electron beam, suitably modulated and controlled, is directed towards the target surface in order to generate a charge pattern thereon. While the charge pattern is being generated on the front surface of the target, the small particles described above are maintained out of the influence of the electrostatic field. After the electrostatic charge pattern has been generated on the front surface of the target surface, the charge particles are cascaded across the rear or reverse side of the target by physically moving both the target and the container, which is holding the particles, so that the force of gravity pulls the particles across the target face. The electrostatic field, which results from the disposition of electrical charges in different discrete areas of the target surface, extends through the thin target member and it exerts an attractive force on the cascading charge particles. This force causes the particles to adhere to the reverse side of the target surface in the charged areas thereof. The adherence of the suitably colored charged particles to the oppositely charged areas of the target surface produces a visible image of the information which had been written on the target surface in the form of an electrostatic charge distribution pattern. The remainder of the developer is returned to its reservoir by gravity. A visible image then may be viewed or photographed through the transparent face plate at the end of the evacuated envelope.

When it is desired to erase the electrostatic image and a visible image, a flood beam of electrons is directed at the charged side of the target surface and the developer is again cascaded over the opposite side of the target. The velocity of the electrons in the flood beam is such, in relation to the secondary emission properties of the insulating material which forms the target surface, that the secondary emission ratio of the target surface is greater than unity under the action of the flood beam. This means that for every incoming electron from the flood beam, more than one electron leaves the target surface until an equilibrium is reached with the conductive coating within the tube. The flood beam, in cooperation with the cascading developer particles, eradicates the electrostatic charge on the target and gravity returns the developer to its reservoir. After the electrostatic charge has been eradicated, the target surface is available for storage of another electrostatic image.

The present invention provides an improvement upon the above described system. In the system described in the above cited reference, the charged particles are cascaded over the target surface by physically moving the target surface and the container which holds the charged particles. A device which relies for its operation upon mechanical motion of this type has obvious disadvantages. Either the entire tube must be mounted in a rotatable mounting or at least the target surface and container which holds the particles must be rotatably mounted. Such mountings are expensive, difiicult to design and manufacture with a high degree of precision, and they generally require a substantial amount of maintenance. The volume required for mounting the tube is considerably increased. In addition, there is the problem of providing high voltage (up to 20 kv.) and all the other requisite voltages to the tube through leads which must be mechanically flexible.

The present invention provides an improved device wherein the target and tube remain stationary. In the system of the present invention the charged particles are cascaded over the surface of the target by means of an electrostatic actuator.

One of the advantages of the present system is that it can provide continuous development in the storage tube. That is, the charged particles can be cascaded over the surface while the electron beam is in the process of writing an image on the opposite surface of the target. One example of where this feature of the invention can be extremely useful is where the device is used to present one image of a multi-page stored document. If one is thumbing through a stored multi-page document, the reader may be able to find that the first few lines indicate that he is not interested in the document and, hence, he need not wait for an entire write-develop cycle before turning to a new page.

Another advantage of the present invention is that it introduces no extraneous magnetic fields into the evacuated envelope. Such fields can be extremely disturbing in all types of cathode ray tubes and electron beam equipment.

Still another advantage is that the power which is supplied to the actuator is at high voltage and low current. It is much easier to pass high voltage and low current through a glass envelope.

Yet other advantages of the present invention is loW cost, an ability to operate at high frequencies, no mechanical fatigue problems, and no problems due to standing wave patterns.

The object of the present invention is to provide an improved electrostatic writing and printing device.

Yet another object of the present invention is to provide an electrostatic writing and printing device wherein neither the target surface nor the tube itself need be mechanically movable.

Still another object of the present invention is to provide an electrostatic writing and printing device which includes a minimum number of mechanically movable parts.

Yet another object of the present invention is to provide an improved means for spreading toner particles over the target face in an electrostatic writing and printing device.

A still further object of the present invention is to provide an electrostatic actuator for propelling toner particles over the target face in an electrostatic writing and printing device.

A still further object of the present invention is to provide an improved electrostatic mechanical actuator.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

FIGURE 1 shows a cutaway side view of a first preferred embodiment of the invention.

FIGURE 2 shows an enlarged cutaway perspective view of the electrostatic actuator.

FIGURE 3 shows an alternate preferred embodiment of the electrostatic actuator.

FIGURE 4 shows a second alternate preferred embodiment of the electrostatic actuator.

The first embodiment shown in FIGURE 1 includes an evacuated chamber 11, a transparent face plate 11e, a write gun 12, an erase gun 22, a target 16, general control circuitry 28, development control circuitry 29, developer particles 21 and 23 and electrostatic actuator 17. In general, the device operates as follows: Write gun 12 forms a pattern of electrostatic charges on side 16a of screen 16. Thereafter, or simultaneously, toner developer particles 21 and 23 which are electrically charged (in a manner which will be explained later) are cascaded across side 1611 of screen 16. Those points on screen 16 which are electrostatically charged attract the oppositely-charged developer particles creating a visible image which can be viewed through transparent face plate He. The image can be eradicated by means of erase gun 22. In order to eradicate an image, erase gun 22 floods screen 16 with electrons at a velocity such that the secondary emission ratio exceeds unity. At the same time the developer particles are cascaded over the screen. The combined action of the electrons from the flood gun and the cascading developer particles eradicates the image.

Evacuated envelope 11, face plate lle, write gun 12, erase gun 22, target 16, particles 21 and 23, and control circuitry 28 may be identical to those described in United States Patent 3,109,062, entitled Electrostatic Writing and Printing Device by Calvin K. Clauer and Jack D. Kuehler, which is assigned to the assignee of the present invention. Hence, these elements will not be described in detail herein.

The present invention includes electrostatic actuator 17 which is used to cascade particles 21 and 23 over screen 16. Chamber 11 and target 16 are mounted so that target 16 is inclined by about five degrees from a vertical position. In this manner the toner particles hit the screen and tumble down its slightly inclined surface, thereby providing .the required cascading action for proper development of the image. Electrostatic actuator 17 includes a relatively thick stationary member 18 and a relatively thin flexible member 20 (see FIGURE 2). Elements 18 and 20 are positioned in a gasket 26 which provides a seal between the components of the electrostatic actuator 17 and (a) target 16, (b) face plate He, and (c) the sides 11s of evacuated chamber 11. The purpose of .this gasket is to prevent the developer from dropping below actuator 17, thereby being lost to the cascading process. Gasket 26 is an electrical insulator and elements 18 and 20 are physically separated by means of three longitudinal strips of electrically insulating material 19.

Two wires 24 and 25 pass through the walls of chamber 11 and are respectively attached to elements 18 and 20. Conventional gaskets 27 are provided at the point where lines 24 and 25 pass through the wall of envelope 11. Lines 24 and 25 are connected to development control circuitry 29. Development control circuitry 29 can apply an alternating voltage of several thousand volts between elements 18 and 20 by means of wires 24 and 25. When an alternating voltage is applied between elements 18 and 20, flexible element 20 is vibrated in synchronization with the voltage, due to the electrostatic forces generated. The vibration of element 20 projects particles 21 and 23 upwards against face plate 16. The amount of current required to drive actuator 17 is very small and hence only a very small magnetic field is generated by actuator 17. For this reason, actuator 17 does not produce any interference with the writing operation.

Elements 18 and 20 are trough shaped so that there is a focusing like action as the particles are projected. As a result, the majority of the particles hit the screen near the .top portion thereof. After hitting the screen the particles cascade down the inclined face of the screen.

Element 20 consists of a sheet of 2% beryllium copper, which is 1 to 3 mils thick. Element 18 consists of a sheet of molybdenum one-eighth of an inch thick. Insulating element 19 consists of Dow Corning 805 silicone resin (trademark). The silicone resin also serves as a bonding agent between elements 18 and 20. Naturally, the composition of the various elements must be chosen so as not to contaminate the evacuated chamber. In a chamber which has an evacuation pump attached thereto, and which is constantly being evacuated, .the problem of contamination is not so severe; however, in a sealed unit more care must be taken in selecting materials.

Toner particles 21 are preferably of a dark color so as to produce a maximum contrast with the insulating member 16 when the particles adhere thereto in response to the electrostatic charge distribution pattern. Particles 23 may be charged in any suitable manner such as by the triboelectric effect. In this case the particles 21 are mixed with other particles 23 of a dissimilar nature so that the agitation of the two dissimilar particles produces opposite electrical charges thereon. The two types of particles utilized may be of any suitable type having the required relationship in the triboelectric series. One example of suitable material is the use of resin or plastic particles 21 and small glass beads or particles 23. The granular plastic particles 21 being electrically insulated retain their electrical charge after agitation with the particles 23 so that they are therefore attracted to the charged areas on target 16.

The write gun 12 and the erase gun 22 are herein shown as being off of the central axis. The purpose of this is to facilitate optical projection of image from target 16 to a large screen. A window (not shown) can be provided in the rear of the tube between write gun 12 and erase gun 22. Light can then be directed through face plate 11e through target 16 and through the window in the rear of tube to a screen (not shown). The image on target 16 could then be viewed on the screen in enlarged form.

FIGURE 3 shows a cross-sectional view of a first alternate embodiment of the present invention. This embodiment eliminates the need for any cement between the various elements. It includes a relatively thin, flexible, electrically-nonconductive element 231 which has a conductive coating 220 and a relatively thick, electricallyconductive element 218. Element 218 has a plurality of holes 215 therein and the toner particles 21 and 23 are positioned on top of element 218. The toner particles 21 and 23 extend into holes 215. An alternating voltage is applied between elements 220 and 218 thereby moving element 220 to provide the required vibrating motion. The ends 232 and 233 of elements 220 and 231 extend around the edges of element 218. A spring 253 is connected between ends 232 and 233 thereby providing tension to hold element 231 in a relatively fixed position around element 218. This embodiment has the advantage that it eliminates the need for any cement thereby eliminating possible contaminants in the evacuated chamber. Insulator 231 may be a sheet of Teflon (trademark) Conductor 220 may consist of an aluminized surface on the sheet of Teflon.

FIGURE 4 shows a cross-sectional view of a second alternate embodiment. This embodiment includes a fixed element 401 and a movable element 402. An alternating voltage is applied between elements 401 and 402 in order to provide the required vibratory motion. Element 402 is attached to element 401 at its end point 402a and 4022). Two strips of insulating material 451 and 452 maintain electrical isolation between elements 401 and 402. Tension is applied to element 402 when it is attached so that it tends to assume a chordal position. Wires 404, 405 and 406 are attached to the central portion of element 402 and tension is applied thereto so that element 402 is pulled into a concave shape. Wires 404, 405 and 406 are attached to a member 409. Member 409 is held away from element 402 by means of compression springs 411, 412, 413 and 414. Compression springs 411 to 414 are attached to element 409 by means of set screws 421, 422, 423 and 424.

Element 409 is made of an electrically insulating plastic such as Teflon (trademark). Hence, by selectively adjusting the amount of tension applied by screws 421 to 424 the shape of element 402 and its spacing from element 401 can be controlled. In this manner the spatial distribution in which particles 21 and 23 are propelled at target 16 can be adjusted, and the efliciency of propulsion can be optimized.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in the form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In a device which forms an electrostatic image on the first side of an insulating target which is located in an evacuated chamber,

a plurality of toner particles,

a movable member adapted to support said toner particles,

a fixed member juxtoposed to said movable member,

means for electrically insulating said movable member from said fixed member, and

means for applying an alternating potential between said members whereby said particles are projected on said target.

2. In a device which forms an electrostatic image on the first side of an insulating target which is located in an evacuated chamber,

a plurality of toner particles,

a movable member positioned within said evacuated chamber on the second side of said target, said movable member adapted to support said toner particles,

a fixed member within said evacuated chamber juxtaposed to said movable member,

means for selectively insulating said members from each other, and

means for applying an alternating potential between said members,

whereby said toner particles are projected on said second side of said target.

3. In a device which forms an electrostatic image on the first side of an insulating target which is located in an evacuated chamber, said target being inclined several degrees from the vertical,

a plurality of toner particles,

a movable member positioned within said evacuated chamber on the second side of said target, said movable member adapted to support said toner particles,

a stationary member within said evacuated chamber juxtaposed to said movable member,

means for selectively insulating said members from each other, and

means for applying an alternating potential between said members,

whereby said toner particles are projected on and cascaded down second side of said target.

4. Apparatus for forming a visible image from an electrical information-bearing signal including:

an evacuated envelope,

a source of electrons in said envelope,

a target member of electrical insulating material disposed at one end of said envelope,

a plurality of electrically charged particles of small size disposed in a chamber on the opposite side of said target member from said electron source,

means for directing said electrons in a beam toward said target member and for sweeping said beam across said target member,

means for modulating the intensity of said directed beam as a function of said electrical signal as said beam sweeps across said target to produce an electrostatic charge distribution pattern on said target surface corresponding to said information, and

an electrosatic actuator within said evacuated chamber for projecting said particles at said target member whereby said charged particles adhere to the charged areas of said target to produce a visible im' age of said charged areas.

ation, by Larson, IRE Transactions, An Audio, vol. AU-4, No. 2, March 1956, pp. 32-36.

JOHN W. CALDWELL, Acting Primary Examiner.

R. K. ECKERT, JR., Assistant Examiner. 

1. IN A DEVICE WHICH FORMS AN ELECTROSTATIC IMAGE ON THE FIRST SIDE OF AN INSULATING TARGET WHICH IS LOCATED IN AN EVACUATED CHAMBER, A PLURALITY OF TONER PARTICLES, A MOVABLE MEMBER ADAPTED TO SUPPORT SAID TONER PARTICLES, A FIXED MEMBER JUXTOPOSED TO SAID MOVABLE MEMBER, MEANS FOR ELECTRICALLY INSULATING SAID MOVABLE MEMBER FROM SAID FIXED MEMBER, AND MEANS FOR APPLYING AN ALTERNATING POTENTIAL BETWEEN SAID MEMBERS WHEREBY SAID PARTICLES ARE PROJECTED ON SAID TARGET. 